1. Field of the Invention
This invention relates to a constant-speed cruising control system for a vehicle, which makes the vehicle cruise at a desired constant speed.
2. Description of the Prior Art
Constant-speed cruising control systems for making vehicles cruise at a desired constant speed are known in the art. For example, in the system disclosed in Japanese Unexamined Patent Publication No. 58(1983)-39312, the vehicle is made to cruise at a desired constant speed by feedback control of the throttle opening according to the difference between the actual vehicle speed and the target vehicle speed (the desired speed). That is, once a target vehicle speed is set, the engine output is controlled on the basis of a feedback signal representing the difference between the actual vehicle speed and the target vehicle speed to match the actual vehicle speed with the target speed without operation of the accelerator pedal.
Some of such constant-speed cruising control systems are provided with a function for accelerating the vehicle by controlling the throttle opening without operation of the accelerator pedal when the target vehicle speed is increased and when the vehicle is to be accelerated to the preceding target speed when the constant-speed cruising control is resumed after interruption. That is, when the target vehicle speed, which the vehicle speed is to match, is to be increased, a setting switch is operated and the vehicle is accelerated until the setting switch is released. The point at which the setting switch is released indicates the target vehicle speed, and feedback control of the vehicle speed is started when the setting switch is released. On the other hand, when a resume switch is operated after the constant-speed cruising control has been interrupted by operation of the brake, the vehicle is accelerated to the previously set target vehicle speed. However, when the vehicle is accelerated by controlling the throttle opening without the driver's operating the accelerator pedal, a problem arises because the acceleration of the vehicle changes according to the conditions under which the vehicle runs, i.e. the running condition, and can therefore deviate from the desired acceleration.
More particularly, the acceleration of the vehicle for a given throttle opening changes with the load on the engine, which load depends on the condition of the road surface, e.g., an ascent or a descent. The acceleration of the vehicle also changes with a driving torque which depends on, for instance, the gear stage of the transmission. Accordingly, the vehicle speed control performance may deteriorate.
Although it has been proposed to take into account the running condition of the vehicle during feedback control of the vehicle speed on the basis of the difference between the actual vehicle speed and the target vehicle speed (See, for example, U.S. patent Application Ser. No. 032,236.), the aforesaid problem, i.e. that the acceleration of the vehicle fluctuates according to the running condition of the vehicle, has not been solved.